The importance of the protection of electronic/electrical devices from being stolen are emphasized since many such a valuable devices are used in hospitals, schools, and laboratories. Various alarm apparatus systems are invented and produced to fulfil the necessity arisen from the experiences that those expensive devices have been lost. There are many different ways to activate an alarm system by using different type of sensors or radio communication devices. Sensors have their inherent lapse in adjusting their sensitivity. If a sensor is adjusted too sensitive, it may cause false alarm while if it is adjusted too loosely, it may miss a triggering of the alarm. Some alarm systems use a motion detector as an alarming initiator. The installation of the detector itself requires a quite of work.
As of U.S. Pat. No. 5,767,771, unplugging a power cord from a outlet arms an alarm system. Lamont""s system is actually activated by a motion sensor or a tempering switch like so many other alarm systems. When the main switch is turned off, the alarm circuit is on the stand by state, and unplugging the power cord does not affect the alarm circuit operation at all. The present system is actually activated by unplugging the power cord irrespective to the state of the main switch. The two systems, present invention and Lamont""s system, are quite different functionally and in physical structure. Present invention and so many other systems are armed by simply turning power switch on. Lamont""s system does not have a mechanism to turn the alarm off, once it is activated. My system will be turned off automatically after the predetermined period of time set by the timer circuit (34). The voltage level detector circuit in this invention detects whether the power cord is plugged or unplugged. Once it detects the disconnection of AC plug, it triggers timer circuit and activates the alarm. Alarming duration is determined by the RC timer circuit. Optionally, alarming duration can be extended until the system is turned off. In U.S. Pat. No. 4,736,195, McMurtry used comparator circuit. However, he used very complicated associated circuit that requires much more difficult design criteria. As of McMurtry system, as long as the main switch (320) is on, the alarm system is not working. The system also requires an AC signal generator, frequency divider, switch circuit. Designing those circuits must go through lots of trial and error process. If the diode (323) is a typical diode structure, it""s forward voltage drop is 0.7 volts. The collector of the transistor (308) does not have much room for the voltage change. It looks simple but hard to set appropriate voltage level without false triggering, especially when AC is superimposed on the DC level.
The arrow headed side of power switch (320) is connected to the power transformer, which is connected in parallel with the appliance circuit. That is why if the switch (320) is closed, the alarm system does not work. Alarm will be on when the flip-flop (324) is on state. As the solution of all the aforementioned shortcomings of an alarm device, the present invention utilizes the method of unplugging power cord for activating an alarm system as explained in the summary part and in the detailed description section.
The present invention is regarding to a burglar alarm means which will be activated by unplugging the power cord connected to an alarm means and an electronic/electrical device in parallel. A voltage level detector circuit is checking the state of the power cord connection constantly. When the cord is unplugged or broken, it will generate a right polarity of output to said alarm apparatus. The alarm activator circuit inverts the detector output to the opposite polarity. Timer circuit is responsible to control the time length how long the alarm will be turned on. The power amplifier amplifies the output current to provide enough power to drive the sound device. The novel feature of this invention is the way activating an alarm device which is securely fastened or attached to an electronic/electrical apparatus. If a person try to move an electronic device from one location to another location, one should unplug the power cord from a power cord outlet. This will also disconnect the AC power from the alarm device and activate it because when the electronic device is unplugged, the alarm device is also unplugged. This method is quite different from the most other features such as motion sensor, infrared sensor, or shock sensor. Those sensing devices are very critical to adjust the sensitivity. The present invention""s method to initiate an alarm condition is very straightforward, plugged or unplugged, no gray area. The sound device can be a speaker, buzzer, horn, chime, or any other noise generating apparatus. This invention will provide an alarm system which is economical to manufacture, easy to use and install, and versatile in it""s function. To conserve the battery voltage, this system uses the CMOS devices for the voltage level detector, alarm activator, and the timer circuit. Optionally, this system uses a voltage charger circuit to charge the battery voltage.